rose mpi-launch: relax requirement for >=1 args.

Should allow use of a command file for some mpi launchers.
rose mpi-launch: tidy up usage.
rose mpi-launch -f FILE: new option.

Support launching with a command file.
rose mpi-launch: add command launch tests.
rose mpi-launch: allow default command file.

If "rose-mpi-launch.rc" exists in $PWD, it will be assumed as the
command file to submit to the launcher program.

More tests are also added in this commit.
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Merge branch 'master' into 28.rose-mpi-launch.file
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Merge branch 'master' into 28.rose-mpi-launch.file

Conflicts:
	etc/rose.conf.example
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch '28.rose-mpi-launch.file' of github.com:matthewrmshin/rose into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file
Merge branch 'master' into 28.rose-mpi-launch.file